An aluminum electrolytic capacitor is commonly produced as follows:
A capacitor element is prepared by winding an aluminum foil. The capacitor element is then impregnated with a driving electrolyte.
The capacitor element is next placed in a cylindrical case, which is made of metals such as aluminum, etc. and which has one end left open. The open end of the case is covered by a disc having a pair of lead terminals and is then sealed.
During the foregoing assembly process, the capacitor element is connected electrically to the pair of lead terminals.
On the other case end (opposite to the open end) a pattern such as an X letter, a Y letter, etc., is cut to serve as an explosion preventive vent.
After assembly of the aluminum electrolytic capacitor has been completed, the capacitor is subjected to an aging process wherein an electric voltage is applied across the pair of lead terminals. In this manner, the capacitor performance is stabilized and also defects such as an internal short circuits, etc., if any, are uncovered prior to shipment.
The aging process has been conducted, as illustrated in FIG. 1(a), with an aluminum electrolytic capacitor 1 supported vertically with one end 2 having an explosion preventive vent positioned at the upper side and another end 4 having lead terminals 3 positioned at the lower side, the pair of lead terminals 3 is inserted to and held by a mating pair of metal clips 6 which are mounted on a support table 5 and made of an elastic electrode material. An electric voltage from a power source 8 is applied to the lead terminals 3 through lead wires 7 which are connected to the clips 6.
This aging process has been employed widely and commonly for the reasons that mounting and removing of the aluminum electrolytic capacitor 1 to and from the support table 5 is easily achieved, especially when done by hand.
However, the conventional aging method of the foregoing has several problems:
One of the problems is contamination of the clips 6. Contamination may be caused by the driving electrolyte flowing out of the case end 2 broken along the cut 9 that was provided as an explosion preventive vent as illustrated in FIG. 1(b). If internal shortcircuiting takes place during the course of aging, electrolyte may flow out of the explosion preventive vent on the case end 2, along the side of the capacitor 1, and down to the clips 6.
An insulating oxide film is formed on the surface of the clips 6 by a reaction between the electrolyte and the clips 6 with a resultant interruption of voltage application to the lead terminals 3 through the clips 6. Thus, the next time an aging process is applied to an aluminum electrolytic capacitor using the contaminated clips 6 coated with the oxide film, aging does not occur.
Thus, once an oxide film is formed on the clips 6, the clips cannot be used again and every time the explosion preventive vent operates, the clips 6 have to be checked (and sometimes replaced) resulting in the necessity of frequent maintenance of the aging equipment and an adverse effect to productivity.
Another defect is the likelihood of a solder layer that was formed by plating on the surface of the lead terminals 3 to be peeled off when the aluminum electrolytic capacitor 1 is mounted on or removed from the clips 6.
This causes a poor connection between the capacitor lead terminals having the plated solder layer peeled off of the surface and the circuit pattern of a printed wiring board when the aluminum electrolytic capacitor is mounted on the printed wiring board by soldering.
Particularly with an aluminum electrolytic capacitor of shorter lead terminal type that was recently put to a practical usage, the gripping force of the clips is intensified to prevent the capacitor from falling off of the clips due to the shorter lead terminals. Intensification of the gripping force may result in peeling of the solder layer formed by plating on the surface of the lead terminals.
Relating to the type of aluminum electrolytic capacitor having a shorter lead terminal, the height of the lead terminals measured from the surface of the capacitor end does not exceed 4.5 mm, in contrast to the conventional ordinary capacitor which has a height of about 6.3 mm.
When the conventional capacitor is mounted on a printed circuit board, the tips of the lead terminals protrude from the bottom surface of the board requiring an extra process of shortening the protruding tips of the lead terminals. With the capacitor of shorter lead terminal type, the tips of the lead terminals do not protrude from the bottom surface of the board, eliminating the necessity of the extra process of shortening the protruding tips of the lead terminals. As a result, more of the aluminum electrolytic capacitors of shorter lead terminal type are recently being used as the most suitable capacitors for mounting on printed wiring boards.
Another problem relates to a difficulty in automating the mounting and removing of the lead terminals to and from the clips since a certain appropriate force is required for performing the aforementioned operations by machine although they are easily performed by hand.